As a stepless transmission provided in an automobile or the like, there is known a belt type stepless transmission including a primary pulley to which drive force of an internal combustion engine is transmitted, and a secondary pulley connected to vehicle wheels. The winding radius of a belt that is wound around this pair of pulleys is changed, thereby continuously changing a transmission gear ratio in a stepless manner.
According to the belt type stepless transmission, the hydraulic pressure in a hydraulic chamber formed in each of the pulleys is changed to adjust the groove width of the pulley, thereby changing the winding radius of the belt, and controlling the transmission gear ratio. Hence, the belt type stepless transmission includes a hydraulic device that controls the hydraulic pressure of hydraulic oil that is supplied to the hydraulic chamber of each pulley. The hydraulic device includes an engine-driven oil pump that pumps the hydraulic oil utilizing drive force of the internal combustion engine. The hydraulic oil that is pumped by the engine-driven oil pump is supplied to the hydraulic chamber of each of the pulleys.
The discharge performance of the engine-driven oil pump changes as the engine rotational speed changes. Hence, when the engine rotational speed is low and the discharge performance is low, the discharge performance of the oil pump becomes insufficient and there is an adverse possibility that a necessary amount of hydraulic oil cannot be supplied to the hydraulic chamber of the pulley.
A hydraulic device described in Patent Document 1 includes a sub-pump in addition to a main pump. When the engine rotational speed is low, hydraulic oil is supplied from both the main pump and the sub-pump so that the necessary amount of hydraulic pressure is secured, and when the engine rotational speed is high, hydraulic oil discharged from the sub-pump is not supplied to the hydraulic chamber and is caused to return to an oil pan. If this configuration is employed, as the engine rotational speed increases, the discharge performance of the main pump increases. When it becomes possible to secure the necessary amount of hydraulic pressure only by hydraulic oil discharged from the main pump, the load on the sub-pump is reduced. It is possible to prevent hydraulic oil from being unnecessarily pumped by the sub-pump, and to prevent hydraulic pressure from becoming insufficient when the engine rotational speed is low.
As a specific configuration for a hydraulic device in which as the discharge performance of the main pump increases, supply paths for hydraulic oil discharged from the sub-pump are switched, it is possible to employ a configuration including a regulator 5 and a check valve 8 as shown in FIG. 9, for example.
As shown in FIG. 9, this hydraulic device includes a main pump 1 and a sub-pump 2 as engine-driven oil pumps. A main passage 3 is connected to the main pump 1, and hydraulic oil that is pumped from the main pump 1 is supplied to a speed-changing hydraulic pressure circuit through the main passage 3 and to a lubricating-hydraulic pressure circuit. A sub-passage 4 that is in communication with the lubricating-hydraulic pressure circuit is connected to the sub-pump 2. The speed-changing hydraulic pressure circuit supplies hydraulic oil to hydraulic chambers of the pulleys. The lubricating-hydraulic pressure circuit supplies hydraulic oil to various portions of the stepless transmission as lubricant oil.
The main passage 3 and the sub-passage 4 include the regulator 5. As shown in FIG. 9, the regulator 5 is provided at a section of the main passage 3 located downstream of a location X, where the speed-changing hydraulic pressure circuit is connected, and its spool valve is always biased in a direction closing the main passage 3 and the sub-passage 4 by a spring 5a. The regulator 5 includes a feedback passage 5b that causes the hydraulic pressure of hydraulic oil supplied to the speed-changing hydraulic pressure circuit to be applied to the spool valve. If the hydraulic pressure of the hydraulic oil supplied to the speed-changing hydraulic pressure circuit increases, a larger hydraulic pressure is applied to the spool valve, and the spool valve is displaced to its opening side against the biasing force of the spring 5a. 
The regulator 5 includes a main port 5c through which hydraulic oil flowing through the main passage 3 passes, and a sub-port 5d through which hydraulic oil flowing through the sub-passage 4 passes. The shapes of the main port 5c and the sub-port 5d are set such that opening areas thereof with respect to the amount of displacement of the spool valve to its opening side change as shown in FIG. 10.
More specifically, when the amount of displacement of the spool valve is significantly small, the main port 5c and the sub-port 5d both close, and the regulator 5 prohibits hydraulic oil from flowing through the main passage 3 and the sub-passage 4. If the hydraulic pressure applied to the spool valve via the feedback passage 5b increases and the amount of displacement of the spool valve to its opening side increases, the opening area of the main port 5c first increases as shown with the solid line in FIG. 10, and the hydraulic oil flows to a section downstream of the regulator 5 through the main passage 3. When the hydraulic pressure applied to the spool valve via the feedback passage 5b further increases and the amount of displacement of the spool valve further increases, the opening area of the sub-port 5d starts increasing, and hydraulic oil flows to a section downstream of the regulator 5 not only through the main passage 3 but also through the sub-passage 4. As shown in FIG. 10, the greater the amount of displacement of the spool valve to its opening side is, the greater the opening areas of the main port 5c and the sub-port 5d are, and the amount of hydraulic oil supplied to the section downstream of the regulator 5 through the main passage 3 and the sub-passage 4 increases. At that time, when the amount of displacement is equal to or greater than a predetermined amount A, the opening area of the sub-port 5d is greater than that of the main port 5c, and the amount of hydraulic oil flowing through the sub-passage 4 via the sub-port 5d becomes greater than the amount of hydraulic oil flowing through the main passage 3 via the main port 5c. 
A linear solenoid 6 that outputs a control hydraulic pressure is connected to the regulator 5. The control hydraulic pressure biases the spool valve to its closing side. By controlling the linear solenoid 6 to control the magnitude of the control hydraulic pressure that biases the spool valve to the closing side, it is possible to change the amount of displacement of the spool valve with respect to the magnitude of the hydraulic pressure applied to the spool valve via the feedback passage 5b, and to control the magnitude of the hydraulic pressure of hydraulic oil supplied to the speed-changing hydraulic pressure circuit.
Further, as shown in FIG. 9, a bypass passage 7 is provided at a section of the main passage 3 located upstream of the location X, where the speed-changing hydraulic pressure circuit is connected. The bypass passage 7 connects, with each other, the main passage 3 and a section of the sub-passage 4 located upstream of the regulator 5. The bypass passage 7 includes the check valve 8. The check valve 8 opens when the hydraulic pressure of hydraulic oil flowing through a section of the bypass passage 7 closer to the sub-passage 4 is greater than the hydraulic pressure of hydraulic oil flowing through a section of the bypass passage 7 closer to the main passage 3, and the check valve 8 permits only a flow of hydraulic oil from the sub-passage 4 to the main passage 3.
Immediately after the internal combustion engine is driven and the main pump 1 and the sub-pump 2 start pumping the hydraulic oil, the engine rotational speed is low. Thus, the hydraulic pressure supplied to the speed-changing hydraulic pressure circuit through the main passage 3 is significantly small. Hence, at that time, the hydraulic pressure applied to the spool valve of the regulator 5 via the feedback passage 5b is significantly small, and both the main port 5c and the sub-port 5d of the regulator 5 close. Therefore, the hydraulic oil discharged from the sub-pump 2 is not supplied to a section downstream of the regulator 5, and the hydraulic pressure in a section of the sub-passage 4 located upstream of the regulator 5 gradually increases. If the hydraulic pressure in a section of the sub-passage 4 located upstream of the regulator 5 becomes higher than the hydraulic pressure in a section of the main passage 3 located upstream of the location X, where the speed-changing hydraulic pressure circuit is connected, the check valve 8 opens and the hydraulic oil discharged from the sub-pump 2 through the bypass passage 7 flows into the main passage 3. As a result, both the hydraulic oil discharged from the main pump 1 and the hydraulic oil discharged from the sub-pump 2 are supplied to the speed-changing hydraulic pressure circuit through the main passage 3. If both the hydraulic oil discharged from the main pump 1 and the hydraulic oil discharged from the sub-pump 2 are supplied to the speed-changing hydraulic pressure circuit in this manner and the hydraulic pressure of the hydraulic oil supplied to the speed-changing hydraulic pressure circuit increases, the hydraulic pressure applied to the spool valve of the regulator 5 via the feedback passage 5b increases, and the main port 5c first opens. Both the hydraulic oil discharged from the main pump 1 and the hydraulic oil discharged from the sub-pump 2 are supplied to the lubricating-hydraulic pressure circuit through the main passage 3.
In the hydraulic device above, immediately after the internal combustion engine is driven and when the discharge performance of the main pump 1 is low, the check valve 8 opens, and both the hydraulic oil discharged from the main pump 1 and the hydraulic oil discharged from the sub-pump 2 are supplied to the hydraulic pressure circuits through the main passage 3.
If the engine rotational speed increases and the discharge performance of the main pump 1 and the sub-pump 2 increases, the hydraulic pressure applied to the spool valve via the feedback passage 5b of the regulator 5 increases, the amount of displacement of the spool valve increases. If the amount of displacement of the spool valve of the regulator 5 increases, the sub-port 5d opens as shown in FIG. 10, and hydraulic oil is supplied to a section of the sub-passage 4 located downstream of the regulator 5. As a result, the hydraulic pressure of hydraulic oil in a section of the bypass passage 7 located closer to the sub-passage 4 decreases. If the hydraulic pressure in that section becomes lower than the hydraulic pressure of hydraulic oil in the bypass passage 7 closer to the main passage 3, the check valve 8 closes. If the discharge performance of the main pump 1 increases and the check valve 8 closes, the hydraulic oil discharged from the sub-pump 2 is not supplied to the speed-changing hydraulic pressure circuit, and is supplied, through the sub-passage 4, to the lubricating-hydraulic pressure circuit located downstream of the regulator 5.
According to the hydraulic device for a stepless transmission including the regulator 5 and the check valve 8, when the hydraulic pressure of hydraulic oil discharged from the main pump 1 increases, the supply paths for hydraulic oil discharged from the sub-pump 2 are automatically switched. That is, according to this configuration, it is possible to automatically switch between the supply paths for hydraulic oil discharged from the sub-pump 2, and to change the load of the sub-pump without providing a sensor for monitoring the hydraulic pressure of hydraulic oil discharged from the main pump 1.
In the meantime, in the hydraulic device for a stepless transmission including the regulator 5 and the check valve 8, when the hydraulic pressure of hydraulic oil supplied to the speed-changing hydraulic pressure circuit is to be significantly increased, the linear solenoid 6 is controlled to increase the magnitude of a control hydraulic pressure that biases the spool valve of the regulator 5 to its closing side. The spool valve of the regulator 5 is displaced to the closing side, and the amount of hydraulic oil flowing through the sub-passage 4 via the main port 5c and the sub-port 5d decreases. As a result, the hydraulic pressure of hydraulic oil in a section of the sub-passage 4 located upstream of the regulator 5 increases, and the check valve 8 opens. Since the check valve 8 opens in this manner, the hydraulic oil discharged from the sub-pump 2 flows through the main passage 3 together with hydraulic oil discharged from the main pump 1, and it is possible to significantly increase the hydraulic pressure of hydraulic oil supplied to the speed-changing hydraulic pressure circuit.
In the hydraulic device above, since the check valve 8 is opened by operating the linear solenoid 6 to increase the hydraulic pressure in the sub-passage 4, it takes time until the check valve 8 opens and the hydraulic pressure increases after the linear solenoid 6 is operated. Hence, when it is required to quickly change the speed as in a case where abrupt acceleration is requested and the hydraulic pressure required by the speed-changing hydraulic pressure circuit abruptly increases, there is an adverse possibility that the hydraulic pressure will not be increased enough in time. The hydraulic pressure supplied to each of the pulleys becomes insufficient as the transmission gear ratio is changed, and slippage is generated on the belt.    Patent Document 1: Japanese Laid-open Patent Publication No. 2003-193819